Twinkle is a free and open-source application for voice communications over Voice over IP (VoIP) protocol. Architecture It is designed for Linux operating systems and uses the Qt toolkit for its graphical user interface. For call signaling it employs the Session Initiation Protocol (SIP). It also features direct IP-to-IP calls. Media streams are transmitted via the Real-time Transport Protocol (RTP) which may be encrypted with the Secure Real-time Transport Protocol (SRTP) and the ZRTP security protocols. Since version 1.3.2 (September 2008), Twinkle supports message exchange and a buddy-list feature for presence notification, showing the online-status of predefined communications partners (provider-support needed). Supported audio formats G.711 A-law: 64 kbit/s payload, 8 kHz sampling rate G.711 μ-law: 64 kbit/s payload, 8 kHz sampling rate G.726: 16, 24, 32 or 40 kbit/s payload, 8 kHz sampling rate GSM: 13 kbit/s payload, 8 kHz sampling rate G.729: 8 kbit/s payload, 8 kHz sampling rate iLBC: 13.3 or 15.2 kbit/s payload, 8 kHz sampling rate Speex narrow band: 15.2 kbit/s payload, 8 kHz sampling rate Speex wide band: 28 kbit/s payload, 16 kHz sampling rate Speex ultra wide band: 36 kbit/s payload, 32 kHz sampling rate See also Comparison of VoIP software List of SIP software List of free and open-source software packages Opportunistic encryption References Free VoIP software Cryptographic software Internet privacy software Secure communication Software that uses Qt KDE software